1. Field of the Invention
This invention concerns the capillary of a wire-bonding apparatus and methods of manufacturing semiconductor devices using wire bonding apparatus having capillaries.
More particularly, this invention concerns the particullar shape of the working face of the capillary.
2. Description of the Prior Art
In wire bonding processes for connecting predetermined positions, such as electrodes formed on a semiconductor chip of a lead frame, and inner lead portions, there are several alternatives. These include thermocompression bonding, ultrasonic wire bonding, etc. Recently, a so-called thermosonic wire bonding, namely a combination of thermocompression and ultrasonic bonding, has been widely used.
FIG. 1 is an expanded cross section view of a conventional capillary 10 in the vicinity of the head portion thereof. In the drawing, H, which is called the hole diameter, refers to the diameter of the bore guide region 2. The symbol .theta.c, which is called the capillary cone angle, is the taper angle of an imaginary right cone of the capillary body 1, including the circular cone capillary side face 3 of the body 1. T is the diameter of a circle formed by the intersection of the imaginary right cone, including the capillary side face 3, and a plane X-X' which comprises the capillary tip surface 4 including capillary tip 5. The diameter T is called the capillary head diameter.
The bore guide portion 2 has chamfer, or is widened towards the tip 5 to form a bore throat region 7. The bore throat region 7 has the shape of a substantially truncated cone, and has a bore throat diameter B at its widest end.
The reference IC represents the degree of the chamfer, and is half of the difference between the bore throat diameter B and the hole diameter H.
The capillary 10 includes a working face 6 having a radius of curvature OR. The curved surface of the working face 6 terminates at point Q on the capillary side face 3. The letters Dw represents a distance from the point Q to the X-X' plane, which is called as working face height. Examples of typical dimensions are H=38 .mu.m, .theta.c=30.degree., T=203 .mu.m, B=64 .mu.m, IC=13 .mu.m, OR=89 .mu.m and Dw=25 .mu.m.
Referring now to FIGS. 2 and 3, a thermosonic wire bonding process of manufacturing a semiconductor device using the conventional capillary is explained.
A bonding wire 17 made of gold (Au) is supplied through the bore guide portion 2. The bonding wire 17 has a ball portion 18 at the end thereof. An aluminium electrode 31, which is formed on a semiconductor chip 30 bonded on a lead frame 20, is previously heated to about 250 .degree. C. The ball portion 18 is positioned on the aluminium electrode 31 (called hereafter as bonding pad). (FIG. 2)
An ultrasonic vibration is applied to the capillary 10 substantially parallel to the face of aluminum bonding pad 31 while a force is applied to the ball 18 by the working face 6. The ball 18 is crushed to form the flattened shape 18A, and to increase the contact area with the pad 31. (FIG. 3)
During this process, any oxidation film which may be formed between the ball 18 and the bonding pad 31 is destroyed. Thus, the fresh or pure faces of the bonding wire and the bonding pad contact each other to form an alloy layer between them. In this way, the Au bonding wire 17 is bonded to the bonding pad 31.
Next, the capillary 10 is raised to a specific level above the bonding pad 31 with the bonding wire feeding through the bore guide portion 2. The capillary 10 and the pad 31 are then moved relative each other for bonding of the bonding wire at another location, such as an inner lead portion of the lead frame (not shown). At the new location, the bonding wire 17 is bonded by the same method described above, and a loop of the bonding wire is formed between the bonding pad and the inner lead portion. Finally, the capillary is raised to cut the bonding wire 17.
There is a market demand for high integration and size reduction of semiconductor devices. Thus, the pitch or distance between the bonding pads formed on a semiconductor chip continually is being reduced. The reduction of the pitch between the bonding pads creates a problem of damage to the bonding wire during the wire bonding process. Namely, the capillary 10 sometimes contacts a neighboring wire (illustrated in a dotted circle 12 in FIG. 4) which was previously bonded. Therefore, there is a practical minimum to the bonding pad pitch. The minimum of the pitch is about 160 .mu.m for the conventional capillary.
To meet the demand of further reduction of the pitch, the present inventors tried to narrow the capillary core angle .theta.c and the capillary head diameter T, as shown in FIG. 5. In the trial, the inventors adopted dimensions, such as .theta.c=15.degree., and T=165 .mu.m, while keeping the hole diameter H, insert chamfer IC, bore throat diameter B and working face height Dw equal to the conventional capillary shown in FIG. 1. In this trial, the radius of curvature ORa of the working face 6a was 70 .mu.m, which is somewhat smaller than in the capillary of FIG. 1.
The working face 6a applied a force to a bonding wire to crush the wire, and to effectuate a bond between the bonding wire and bonding pad or lead frame. Thus, the crushed shape of the wire was determined according to the shape of the working face. Therefore, by reducing the radius of curvature, the thickness of the bonding wire remaining on the lead frame increased sharply according to the shape of the working face of the capillary. In other words, the area supplied with the force by the capillary to achieve the bond became narrow by reducing the radius of curvature of the working face. Thus, the bondability became poor. The deterioration of the bondability can be fatal for a semiconductor device.